This invention relates to a solid state image pick-up apparatus and more particularly apparatus for decreasing a DC offset current of MOS type solid state image pick-up apparatus and for decreasing fixed pattern image caused by the DC offset current.
For the sake of description, in the following description, electrons are used as a signal charge, but it will be clear that holes can also be used as the signal charge provided that potential relation and conductivity type are reversed.
FIGS. 1A and 1B of the accompanying drawing shows the construction of a MOS type solid state image pick-up apparatus constituted by a plurality of photo diodes arranged in a matrix, vertical switching MOSFETs 9 (hereinafter called photogate circuits) and horizontal switching MOSFETs 4 (hereinafter called SMOS) associated with respective photodiodes 3 for the purpose of reading out signals stored therein, a vertical shift register 2 and a horizontal shift register 1 which are provided for the purpose operating these switching MOSFETs in a predetermined order, a vertical signal line 6 and a horizontal signal line 5 adapted to transmit signals. Transmission lines 8 are provided for the purpose of transmitting switching pulses to the gate electrodes of the SMOSs from the shift register 1, the transmission lines 8 being connected to output terminals 7. Switching pulses are transmitted to the gate electrodes of the photogate circuits 9 from the shift register 2 through transmission lines 10.
In the solid state image pick-up device, various elements described above are formed on semiconductor layers 12, 15 and 17 on a semiconductor substrate, not shown. The semiconductor layers on which the shift registers 1 and 2 and the photodiode array are formed respectively are electrically isolated from each other. The reason for providing the semiconductor layers and for electrically isolating them are discussed in detail in N. Koike et al paper, I.E.E.E. Trans. Electron Devices, ED- 27, No. 8, pages 1676-1681, August 1980. For the purpose of decreasing noise and the impedances of the semiconductor layers, the horizontal signal lines 5 and the SMOSs4 shown in FIG. 1 are formed on the semiconductor layer of the diode array.
In the solid state image pick-up apparatus utilizing semiconductor layers, the fact that the semiconductor layer 17 for the vertical shift register 2 is electrically isolated from the semiconductor layer 12 for the photodiodes is utilized to forwardly bias the semiconductor layer 12 of the photodiode array for the purpose of preventing blooming of the photodiode array thereby making the gate voltage which turns OFF photogates 9 to be negative with respect to the voltage of the semiconductor layer 12 so as to render the gates to the charge accumulation state. The object and effect of this method are disclosed in detail in U.S. Pat. No. 4,223,330 invented by Koike et al.
However, it was recently found that the MOS type solid state image pick-up tube having excellent blooming suppression effect involves the following serious problems.
Firstly, when the apparatus is driven, independently of an electric signal caused by a photoinformation, a definite DC offset current flows. In apparatus utilizing electrons as signal carriers this current is in a direction of flowing out of the apparatus, but its direction is opposite to that of a generation current, normally termed as dark current. Further, this current does not result in a remarkable temperature change as the generation current.
Secondly, the DC offset current varies in a predetermined range in the horizontal direction. This variation is not caused by the charge and discharge of a capacitor as in the fixed pattern noise caused by flowing in and out of electric charge in the SMOSs which has been considered in the prior art MOS type solid state image pick-up apparatus but is a variation in direct current, so that it is impossible to eliminate such current by integrating varying charge. This greatly degraded signal to noise ratio (S/N) under a low light intensity condition, thus resulting in a serious problem for practical apparatus. The fixed pattern noise caused by charging and discharging of the capacitor described above, and a method of eliminating the noise are discussed in detail in M. Aoki et al paper, I.E.E.E. ISSOCC dig. Tech. Papers, 1980, pages 26 and 27, and S. Ohba et al paper, I.E.E.E. Trans. Electron Devices, ED- 27, No. 8, pages 1682-1687, August 1980.
The reason of the above described phenomenon will be described with reference to FIG. 2 in which 21 designates a n type semiconductor substrate, 22 a p type semiconductor layer on which photodiodes and SMOSs are formed. In this example, the p type semiconductor layer comprises a p type wafer on the n-type substrate. Reference numeral 23 designates a n.sup.+ layer for forming a photodiodes, 24 a P.sup.+ layer for increasing a signal accumulation capacitance, 25 a drain electrode for reading out a signal stored in the photodiode, 28 a photo gate electrode, 30 a vertical gate line, 31 a vertical signal line, 29 a gate electrode of a SMOS, 26 and 27 are source and drain electrodes respectively of a SMOS, 32 a signal line for transmitting a pulse from the horizontal shift register to the SMOS gate electrode, and 23 a horizontal signal line.
Regarding the potential relation of the apparatus shown in FIG. 2, when the low level of a pulse applied to the photogate (the voltage in an OFF state) is made to be 0 V (reference), about 1 V (V.sub.WPD), is impressed upon a well 22, more than 1 V (V.sub.SUB) upon the n type substrate 21, 3 to 5 V acting as a video voltage upon signal lines 31 and 32, a high level voltage of 7 V (V.sub.SMOS.H) and a pulse of low level of 0 V (H.sub.SMOS, L) upon the gate electrode 29 of the SMOS.
When the elements are driven, direct current I.sub.D1 flows as shown by an arrow 34 which is read by an ammeter 35. As above described, this direct current flows in a direction opposite to that of the generation current. When DC voltage of 0 to 10 V is applied to the gate electrode of the SMOS by stopping the operation of the horizontal shift register the direct current I.sub.D1 decreases so that no fixed pattern noise caused by the variation of this direct current appears. When the drive frequency of the horizontal shift register is increased or decreased, the the direct current I.sub.D1 also increases or decreases.
As the quantity of charge induced beneath the gate electrode of the SMOS is increased, that is as the voltage V.sub.SMOS.H -V.sub.V is increased, the direct current I.sub.D1 increases, when V.sub.SMOS.H represents the gate voltage when the SMOS is ON.
From the foregoing description, it can be assumed that the DC offset current described above is a charge pump current of the SMOS. When read with ammeters 36 and 37 shown in FIG. 2, it will be noted that the current is substantially equal to the current I.sub.D2 flowing through ammeter 37. In other words, most of the charge pump current flows through the well 22. Since the depth of the well 22 is shallow that is less than 10 microns, if it were assumed that the current is caused by the discharge of the charge beneath the gate electrode like an ordinary charge pump current, the current would flow towards the substrate 21 due to bipolar effect. For this reason, it is considered that the observed charge pump current would be a recombination current produced by the recombination of electrons 38 on the sides of signal lines 31, 33 and holes 39 on the side of the semiconductor layer 22 through an interface at an interface level between Si and SiO.sub.2 beneath the gate electrode of the SMOS as shown in FIG. 2.
The interface level is governed by the quality of the steps of manufacturing the image pick-up apparatus and can not be decreased readily. The fixed pattern noise caused by the variation in the direct current described above decreases the sensitivity of the pick-up device that is a television camera, thus greatly degrading the quality of the picture.